Effective conjugation and Raman intensities in oligo(para-phenylene)s: A microscopic view from first-principles calculations

Abstract

Electron-phonon coupling in oligo(para-phenylene)s is addressed in terms of the off-resonance Raman intensities of two distinct modes at 1220 and 1280 cm(-1). On the basis of Albrecht's theory, vibrational coupling and Raman intensities are calculated from first-principles quantum-chemical methods. A few-state model is used to evaluate the dependence of the mode intensities on oligomer length, planarity, and excitation wavelength. The link between electron delocalizationconjugation and Raman intensities is highlighted. Extending on prior studies, the present work focuses on providing an in-depth understanding of the origin of this correlation in addition to reproducing experimental findings. The model applied here allows us to interpret the results on a microscopic, quantum-mechanical basis and to relate the observed trends to the molecular orbital structure and nature of the excited states in this class of materials. We find quantitative agreement between the results of the calculations and those of measurements performed on oligo(para-phenylene)s of various chain lengths in the solid state and in solution.

Chain length effects of p-oligophenyls with comparison of benzene by Raman scattering

“…This also shows that the 1220 cm 1 mode, which is associated with the delocalized orbitals, is more strongly pre-resonance enhanced than the 1280 cm 1 mode as the exciting laser approaches resonance with the lower energy delocalized band. 37 In Fig. 4(b), we show the ratio of the intensities of the 1220 cm 1 mode and 1280 cm 1 mode for POPs containing between three and six repeat units as a function of the laser wavelength.…”

“…[33][34][35] The ratio of the intensities of these two peaks is also an indicator of the inter-ring torsion angle. [36][37][38][39] In PPP materials, all the molecular orbitals can be divided into delocalized orbitals and localized orbitals. Theoretical calculations 37,44 further showed that the delocalized orbitals are mainly associated with the C atoms on the long molecular axis and strongly sensitive to the conjugation (chain length and planarity), whereas the localized orbitals are mainly contributed by the off-axis C atoms and are far less susceptible to conjugation effects.…”

“…[36][37][38][39] In PPP materials, all the molecular orbitals can be divided into delocalized orbitals and localized orbitals. Theoretical calculations 37,44 further showed that the delocalized orbitals are mainly associated with the C atoms on the long molecular axis and strongly sensitive to the conjugation (chain length and planarity), whereas the localized orbitals are mainly contributed by the off-axis C atoms and are far less susceptible to conjugation effects. In addition, the 1220 cm 1 mode is interpreted to be more closely related to the delocalized state than the localized state, and the 1280 cm 1 mode is the opposite.…”

“…32 For POPs with a long C-C chain length, the ratio of the integral intensity of the 1280 cm 1 and 1220 cm 1 modes in the Raman spectra has often been used to distinguish the chain lengths between different compounds. [33][34][35] It thus serves as an indicator of the planarization of molecules at ambient conditions 36,37 or high pressures. 38,39 Meanwhile, the features of the two peaks at around 1600 cm 1 is also particular.…”

“…The results are compared with previously published data. [33][34][35][36][37][38][39] A new method is introduced to estimate the chain length of POPs and PPP materials.…”